Video decoding method and apparatus using the same

ABSTRACT

A video decoding method according to an embodiment of the present invention may include determining a type of a filter to be applied to a first-layer picture which a second-layer picture as a decoding target refers to; determining a filtering target of the first-layer picture to which the filter is applied; filtering the filtering target based on the type of the filter; and adding the filtered first-layer picture to a second-layer reference picture list. Accordingly, the video decoding method and an apparatus using the same may reduce a prediction error in an upper layer and enhance encoding efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

This application is continuation application of U.S. application Ser.No. 16/013,419, filed on Jun. 20, 2018, which is a continuationapplication of U.S. application Ser. No. 15/343,887, filed on Nov. 4,2016, which is a continuation application of U.S. application Ser. No.14/326,232, filed on Jul. 8, 2014, and the present application claimsthe benefit of priority of Korean Patent Applications No.10-2013-0080033 filed on Jul. 9, 2013 and No. 10-2014-0066012 filed onMay 30, 2014 which is incorporated by reference in its entirety herein.

TECHNICAL FIELD

The present invention relates to video encoding and decoding, and moreparticularly, to a method and apparatus for encoding and decoding avideo supporting a plurality of layers in a bit stream.

BACKGROUND ART

In recent years, as high definition (HD) broadcast services arespreading domestically and globally, a large number of users are gettingused to high-resolution and high-quality videos and accordinglyinstitutions put spurs to the development of next-generation videodevices. Also, with growing interest in ultrahigh-definition (UHD)services having a resolution four times higher than HDTV, compressiontechniques for higher-quality videos are needed.

For video compression, there may be used an inter prediction techniqueof predicting pixel values included in a current picture from temporallyprevious and/or subsequent pictures of the current picture, an intraprediction technique of predicting pixel values included in a currentpicture using pixel information in the current picture, or an entropyencoding technique of assigning a short code to a symbol with a highappearance frequency and assigning a long code to a symbol with a lowappearance frequency.

Video compression technology may include a technique of providing aconstant network bandwidth in restricted operating environments ofhardware without considering variable network environments. However, tocompress video data used for network environments involving frequentchanges of bandwidths, new compression techniques are required, whereina scalable video encoding/decoding method may be employed.

DISCLOSURE Technical Problem

An aspect of the present invention is to provide a videoencoding/decoding method using inter-layer filtering, and an apparatususing the same.

Another aspect of the present invention is to provide a video decodingmethod of adaptively applying one or more filters to a lower-layerpicture and adding the picture to a reference picture list for an upperlayer to reduce a prediction error in the upper layer and to improvecoding efficiency, and an apparatus using the same.

Still another aspect of the present invention is to provide a videodecoding method for enhancing coding efficiency without increasing thereference picture list, and an apparatus using the same.

Technical Solution

An embodiment of the present invention provides a video decoding methodsupporting a plurality of layers, the video decoding method includingdetermining a type of a filter to be applied to a first-layer picturewhich a second-layer picture as a decoding target refers to;

determining a filtering target of the first-layer picture to which thefilter is applied; filtering the filtering target based on the type ofthe filter; and adding the filtered first-layer picture to asecond-layer reference picture list.

The determining of the type of the filter may determine to apply a fixedfilter to the first-layer picture.

The fixed filter may be a default filter having a preset filtercoefficient set, and when the default filter is employed, a sample at aninteger position may be unfiltered.

The fixed filter may be an alternative filter having a preset filtercoefficient set, and when the alternative filter is employed, a sampleat an integer position may be filtered.

The alternative filter may be applied to the first-layer picture whichis subjected to the default filter.

The determining of the type of the filter may further include receivingand decoding a flag signal about whether to apply the alternativefilter.

The determining of the type of the filter may determine to adaptivelyselect and apply one or more filters to the first-layer picture.

The determining of the type of the filter may include determining toapply a default filter having a preset filter coefficient set to thefirst-layer picture and determining whether to apply an alternativefilter different from the default filter to a sample at an integerposition in the first-layer picture, and the determining whether toapply the alternative filter may receive and decode a flag signalindicating whether to apply the alternative filter.

The determining of the type of the filter may include determining toapply a default filter having a preset filter coefficient set to thefirst-layer picture and determining whether to apply an alternativefilter different from the default filter to a sample at an integerposition in the first-layer picture, and the determining whether toapply the alternative filter may include calculating a samplecorrelation in each block unit with a predetermined size of thefirst-layer picture; and determining whether to the alternative filterto the first-layer picture based on the correlation.

The determining whether to apply the alternative filter may determine toapply the alternative filter when block activity of the block unit basedon horizontal activity and vertical activity of the block unit is apredetermined threshold or greater.

Another embodiment of the present invention provides a video decodingapparatus supporting a plurality of layers, the video decoding apparatusincluding a decoding module to decode a first-layer picture which asecond-layer picture as a decoding target refers to; a filter module todetermine a type of a filter to be applied to the first-layer picture,to determine a filtering target of the first-layer picture to which thefilter is applied, and to filter the filtering target based on the typeof the filter; and a prediction module to add the filtered first-layerpicture to a second-layer reference picture list.

Advantageous Effects

According to an embodiment of the present invention, there are provideda video encoding/decoding method using inter-layer filtering, and anapparatus using the same.

Also, there are provided a video decoding method of adaptively applyingone or more filters to a lower-layer picture and adding the picture to areference picture list for an upper layer to reduce a prediction errorin the upper layer and to improve coding efficiency, and an apparatususing the same.

In addition, there are provided a video decoding method for enhancingcoding efficiency without increasing the reference picture list, and anapparatus using the same.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a videoencoding apparatus according to an exemplary embodiment.

FIG. 2 is a block diagram illustrating a configuration of a videodecoding apparatus according to an exemplary embodiment.

FIG. 3 is a conceptual diagram schematically illustrating a scalablevideo coding structure using a plurality of layers according to anexemplary embodiment of the present invention.

FIG. 4 illustrates an example of constructing a reference picture list.

FIG. 5 is a flowchart illustrating a video processing method accordingto the present invention.

FIG. 6 illustrates pictures in a plurality of layers according to thepresent invention.

FIG. 7 illustrates block samples for calculating horizontal activity andvertical activity according to the present invention.

FIG. 8 illustrates POCs of first-layer and second-layer picturesaccording to the present invention.

FIG. 9(A) and FIG. 9(B) illustrate a reference picture list according tothe present invention.

FIG. 10 illustrates a reference picture list for a P slice according toan exemplary embodiment of the present invention.

FIG. 11(A), FIG. 11(B) and FIG. 11(C) illustrate a reference picturelist for a B slice according to an exemplary embodiment of the presentinvention.

MODE FOR INVENTION

Hereinafter, embodiments of the present invention are described indetail with reference to the accompanying drawings. In describing theembodiments of the present invention, a detailed description of relatedknown elements or functions will be omitted if it is deemed to make thegist of the present invention unnecessarily vague.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, the element can be directlyconnected or coupled to another element or intervening elements. Also,when it is said that a specific element is “included,” it may mean thatelements other than the specific element are not excluded and thatadditional elements may be included in the embodiments of the presentinvention or the scope of the technical spirit of the present invention.

Although the terms “first,” “second,” etc. may be used to describevarious elements, these elements should not be limited by these terms.These terms are used only to distinguish one element from anotherelement. For example, a first element may be named a second elementwithout departing from the scope of the present invention. Likewise, asecond element may be named a first element.

Although components described in the embodiments of the presentinvention are independently illustrated in order to show differentcharacteristic functions, such a configuration does not indicate thateach component is constructed by a separate hardware constituent unit orsoftware constituent unit. That is, each component includes individualcomponents that are arranged for convenience of description, in which atleast two components may be combined into a single component or a singlecomponent may be divided into a plurality of components to performfunctions. It is to be noted that embodiments in which some componentsare integrated into one combined component and/or a component is dividedinto multiple separate components are included in the scope of thepresent invention without departing from the essence of the presentinvention.

Some constituent elements are not essential to perform the substantialfunctions in the invention and may be optional constituent elements formerely improving performance. The present invention may be embodied byincluding only constituent elements essential to implement the spirit ofthe invention other than constituent elements used for merely improvingperformance. A structure including only the essential constituentelements other than optional constituents used for merely improvingperformance also belongs to the scope of the present invention.

FIG. 1 is a block diagram illustrating a configuration of a videoencoding apparatus according to an exemplary embodiment. A scalablevideo encoding/decoding method or apparatus may be realized by extensionof a general video encoding/decoding method or apparatus that does notprovide scalability, and the block diagram of FIG. 1 illustrates anexample of a video encoding apparatus which may form a basis of ascalable video encoding apparatus.

Referring to FIG. 1, the video encoding apparatus 100 includes a motionestimation module 111, a motion compensation module 112, an intraprediction module 120, a switch 115, a subtractor 125, a transformmodule 130, a quantization module 140, an entropy encoding module 150,an dequantization module 160, an inverse transform module 170, an adder175, a filter module 180, and a reference picture buffer 190.

The video encoding apparatus 100 may encode an input picture images inan intra mode or an inter mode and output a bitstream. Intra predictionmeans an intra-picture prediction, and inter prediction means aninter-picture prediction. In the intra mode, the switch 115 is shiftedto ‘intra,’ and in the inter mode, the switch 115 is shifted to ‘inter.’The video encoding apparatus 100 may generate a prediction block for aninput block of the input picture and then encode a difference betweenthe input block and the prediction block.

In the intra mode, the intra prediction module 120 may perform a spatialprediction by using a pixel value of a pre-encoded block around acurrent block to generate a prediction block.

In the inter mode, the motion estimation module 111 may obtain a regionwhich is most matched with the input block in the reference picturestored in the reference picture buffer 190 during a motion estimationprocess to derive a motion vector. The motion compensation module 112may perform motion compensation using the motion vector and thereference picture stored in the reference picture buffer 190, therebygenerating the prediction block.

The subtractor 125 may generate a residual block based on the differencebetween the input block and the generated prediction block. Thetransform module 130 may transform the residual block to output atransform coefficient. The quantization module 140 may quantize thetransform coefficient according to a quantization parameter to output aquantized coefficient.

The entropy encoding module 150 may entropy-encode a symbol according toprobability distribution based on values derived by the quantizationmodule 140 or an encoding parameter value derived in encoding, therebyoutputting a bitstream. Entropy encoding is a method of receivingsymbols having different values and representing the symbols as adecodable binary sequence or string while removing statisticalredundancy.

Here, a symbol means a syntax element as an encoding/decoding target, acoding parameter, a value of a residual signal, or the like. A codingparameter, which is a parameter necessary for encoding and decoding, mayinclude information encoded by the encoding apparatus and transferred tothe decoding apparatus, such as a syntax element, and information to beinferred during an encoding or decoding process and means informationnecessary for encoding and decoding a picture. The coding parameter mayinclude, for example, values or statistics of an intra/inter predictionmode, a displacement/motion vector, a reference picture index, a codingblock pattern, presence and absence of a residual signal, a transformcoefficient, a quantized transform coefficient, a block size and blockpartition information. A residual signal may denote a difference betweenan original signal and a prediction signal, a transformed signal of thedifference between the original signal and the prediction signal, or atransformed and quantized signal of the difference between the originalsignal and the prediction signal. The residual signal may be referred toas a residual block in a block unit.

When entropy encoding is applied, a symbol having a high probability isallocated a small number of bits and a symbol having a low probabilityis allocated a large number of bits in representation of symbols,thereby reducing a size of bit strings for symbols to be encoded.Accordingly, entropy encoding may enhance compression performance ofvideo encoding.

For entropy encoding, encoding methods, such as exponential Golomb,context-adaptive variable length coding (CAVLC) and context-adaptivebinary arithmetic coding (CABAC), may be used. For example, a table usedfor performing entropy encoding, such as a variable length coding/code(VLC) table, may be stored in the entropy encoding module 150, and theentropy encoding module 150 may perform entropy encoding using thestored VLC table. In addition, the entropy encoding module 150 mayderive a binarization method of a target symbol and a probability modelof a target symbol/bin and perform entropy encoding using the derivedbinarization method or probability model.

The quantized coefficient may be dequantized by the dequantizationmodule 160 and inversely transformed by the inverse transform module170. The dequantized and inversely transformed coefficient is added tothe prediction block by the adder 175, thereby generating areconstructed block.

The reconstructed block is subjected to the filter module 180, and thefilter module 180 may apply at least one of a deblocking filter, asample adaptive offset (SAO), and an adaptive loop filter (ALF) to thereconstructed block or a reconstructed picture. The reconstructed blockobtained via the filter module 180 may be stored in the referencepicture buffer 190.

FIG. 2 is a block diagram illustrating a configuration of a videodecoding apparatus according to an exemplary embodiment. As describedabove in FIG. 1, a scalable video encoding/decoding method or apparatusmay be realized by extension of a general video encoding/decoding methodor apparatus that does not provide scalability, and the block diagram ofFIG. 2 illustrates an example of a video decoding apparatus which mayform a basis of a scalable video decoding apparatus.

Referring to FIG. 2, the video decoding apparatus 200 includes anentropy-decoding module 210, a dequantization module 220, an inversetransform module 230, an intra prediction module 240, a motioncompensation module 250, a filter module 260, and a reference picturebuffer 270.

The video decoding apparatus 200 receives an input bitstream output fromthe encoding apparatus and decodes the bitstream in an intra mode orinter mode to output a reconstituted picture, that is, a reconstructedpicture. In the intra mode, a switch may be shifted to ‘intra,’ and inthe inter mode, the switch may be shifted to ‘inter. The video decodingapparatus 200 may obtain a residual block reconstructed from the inputbit stream, generate a prediction block, and add the residual block andthe prediction block to generate a reconstituted block, that is, areconstructed block.

The entropy decoding module 210 may entropy-decode the input bitstreamaccording to probability distribution to generate symbols including asymbol in a form of a quantized coefficient. Entropy decoding is amethod of receiving a binary sequence to generate symbols. The entropydecoding method is similar to the aforementioned entropy encodingmethod.

The quantized coefficient is dequantized by the dequantization module220 and inversely transformed by the inverse transform module 230,thereby generating a reconstructed residual block.

In the intra mode, the intra prediction module 240 may perform a spatialprediction by using a pixel value of a pre-encoded block around acurrent block to generate a prediction block. In the inter mode, themotion compensation module 250 may perform motion compensation using amotion vector and a reference picture stored in the reference picturebuffer 270, thereby generating a prediction block.

The reconstructed residual block and the prediction block are added byan adder 255, and the added blocks are subjected to the filter module260. The filter module 260 may apply at least one of a deblockingfilter, an SAO, and an ALF to the reconstructed block or thereconstructed picture. The filter module 260 outputs the reconstitutedpicture, that is, the reconstructed picture. The reconstructed picturemay be stored in the reference picture buffer 270 to be used for interprediction.

Components directly related to video decoding among the entropy decodingmodule 210, the dequantization module 220, the inverse transform module230, the intra prediction module 240, the motion compensation module250, the filter module 260 and the reference picture buffer 270 includedin the video decoding apparatus 200, for example, the entropy decodingmodule 210, the dequantization module 220, the inverse transform module230, the intra prediction module 240, the motion compensation module 250and the filter module 260, may be defined as a decoder or a decodingunit, separately from the other components.

In addition, the video decoding apparatus 200 may further include aparsing module (not shown) to parse information about an encoded videoincluded in the bit stream. The parsing module may include the entropydecoding module 210 or be included in the entropy decoding module 210.The parsing module may be configured as one component of the decodingmodule.

FIG. 3 is a conceptual diagram schematically illustrating a scalablevideo coding structure using a plurality of layers according to anexemplary embodiment of the present invention. In FIG. 3, Group ofPicture (GOP) denotes a picture group, that is, a group of pictures.

In order to transmit video data, a transmission medium is needed, andperformance thereof is different by each transmission medium accordingto various network environments. For application to various transmissionmedia or network environments, a scalable video coding method may beprovided.

The scalable video coding method is a coding method which utilizestexture information, motion information, residual signals betweenlayers, or the like to remove redundancy between layers, thus improvingencoding/decoding performance. The scalable video coding method mayprovide scalability in various spatial, temporal, and quality aspectsaccording to ambient conditions such as a transmission bit rate, atransmission error rate, and a system resource.

Scalable video coding may be performed by using a multi-layer structureso as to provide a bitstream applicable to various network situations.For example, the scalable video coding structure may include a baselayer in which video data is compressed and processed using a generalvideo encoding method, and also include an enhancement layer in whichvideo data is compressed and processed using both coding information ofthe base layer and a general video encoding method.

Here, a layer refers to a set of pictures and bitstreams that areclassified according to a spatial aspect (for example, picture size), atemporal aspect (for example, encoding order, picture output order, andframe rate), picture quality, complexity, or the like. Further, the baselayer may mean a lower layer or a reference layer, and the enhancementlayer may mean an upper or higher layer. A plurality of layers may havedependency on each other.

Referring to FIG. 3, for example, the base layer may be defined bystandard definition (SD), 15 Hz frame rate, and 1 Mbps bit rate, a firstenhancement layer may be defined by high definition (HD), 30 Hz framerate, and 3.9 Mbps bit rate, and a second enhancement layer may bedefined by 4K-ultra high definition (UHD), 60 Hz frame rate, and 27.2Mbps. These formats, frame rates and bit rates are provided only forillustrative purposes and may be changed and modified as needed. Also, anumber of used layers may change depending on circumstances, withoutbeing limited to the present embodiment.

For instance, when a transmission bandwidth is 4 Mbps, the firstenhancement layer HD may be transmitted at a frame rate reduced to 15 Hzor lower. The scalable video coding method may provide spatial,temporal, and quality scalabilities using the method described abovewith reference to FIG. 3.

Scalable video coding refers to scalable video encoding in encoding, andto scalable video decoding in a decoding.

The present invention relates to a process of encoding/decoding a videoincluding a plurality of layers or views, wherein the plurality oflayers or views may be expressed as first, second, third and n-th layersor views. Although the following description will be made with referenceto a picture including a first layer and a second layer, the sameprocess may be applied to pictures including two or more layers orviews. The first layer may be represented as a base layer, and thesecond layer as an upper layer. Further, the first layer may be alsorepresented as a reference layer, and the second layer as an enhancementlayer.

A picture/block in the first layer (hereinafter, also referred to as“first-layer picture/block,” the same rule applied throughout)corresponding to a second-layer picture/block may be adjusted to a sizeof the second-layer picture/block. That is, if a size of the first-layerpicture/block is smaller than the size of the second-layerpicture/block, the first-layer picture/block may be scaled usingup-sampling or re-sampling.

The first-layer picture may be added to a reference picture list for thesecond layer and used for encoding/decoding a second-layer video. Here,the second layer may be subjected to prediction and encoding/decodingusing the first-layer picture in the reference picture list, as ingeneral inter prediction.

A block for encoding/decoding may have a square shape with an N×N size,for example, 4×4, 8×8, 16×16, 32×32 and 64×64, or a rectangular shapewith an N×M size, for example, 4×8, 16×8 and 8×32, and a block unit maybe at least one of a coding block (CB), a prediction block (PB) and atransform block (TB), which may have different sizes.

Hereinafter, a method of generating a prediction block, that is, aprediction signal, of an encoding/decoding target block (“current block”or “target block”) in an upper layer will be described in a method ofencoding and decoding a scalable video, that is, a video using amulti-layer structure. The following method or apparatus may begenerally applied to both an encoding apparatus and a decodingapparatus.

In inter prediction, prediction of the current block may be generallyperformed based on a reference picture, which is at least one ofprevious and subsequent pictures of a current picture. A picture usedfor prediction of the current block is referred to as a referencepicture or reference frame.

The reference picture is specified by a reference picture index refIdx,and a region in the reference picture is specified by a motion vector.

In inter prediction, the prediction block for the current block may begenerated by selecting the reference picture and a reference block inthe reference picture corresponding to the current block.

In inter prediction, the encoding apparatus and the decoding apparatusmay derive motion information on the current block and perform interprediction and/or motion compensation based on the derived motioninformation. Here, the encoding apparatus and the decoding apparatus usemotion information on a reconstructed neighboring block and/or acollocated block in an already reconstructed collocated picturecorresponding to the current block, thereby improving encoding/decodingefficiency.

Here, the reconstructed neighboring block, which is a block in thecurrent picture reconstructed via encoding and/or decoding, may includea block adjacent to the current block and/or a block positioned on anouter corner of the current block. Further, the encoding apparatus andthe decoding apparatus may determine a predetermined relative positionbased on a block present at a position spatially corresponding to thecurrent block within the collocated picture and derive the collocatedblock based on the predetermined relative position (internal and/orexternal position of the block present at the position spatiallycorresponding to the current block). For instance, the collocatedpicture may be one picture among reference pictures included in thereference picture list.

In inter prediction, the prediction block with a minimum residual signalfrom the current block and a minimum-size motion vector may begenerated.

Meanwhile, methods of deriving motion information may vary according toa prediction mode of the current block. An advanced motion vectorpredictor (AMVP) mode, a merge mode, or the like may be used as aprediction mode for inter prediction.

For example, when the AMVP mode is employed, the encoding apparatus andthe decoding apparatus may generate a motion vector candidate list byusing a motion vector of the reconstructed neighboring block and/or amotion vector of the collocated block. That is, the motion vector of thereconstructed neighboring block and/or the motion vector of thecollocated block may be used as motion vector candidates. The encodingapparatus may transmit a prediction motion vector index indicating anoptimal motion vector selected among the motion vector candidatesincluded in the list to the decoding apparatus. In this case, thedecoding apparatus may select a prediction motion vector of the currentblock, using the motion vector index, among the motion vector candidatesincluded in the motion vector candidate list.

The encoding apparatus may calculate a motion vector difference (MVD)between a motion vector of the current block and the prediction motionvector, encode the MVD and transmit the MVD to the decoding apparatus.Here, the decoding apparatus may decode the received MVD and adds theMVD to the prediction motion vector to obtain the motion vector of thecurrent block.

The encoding apparatus may also transmit the reference picture indexindicating the reference picture to the decoding apparatus.

The decoding apparatus may predict the motion vector of the currentblock using motion information on neighboring blocks and derive themotion vector of the current block using a residual received from theencoding apparatus. The decoding apparatus may generate the predictionblock for the current block based on the derived motion vector andinformation of the reference picture index received from the encodingapparatus.

Alternatively, when the merge mode is employed, the encoding apparatusand the decoding apparatus may be generate a merge candidate list usingmotion information on the reconstructed neighboring block and/or motioninformation on the collocated block. That is, when the motioninformation on the reconstructed neighboring block and/or on thecollocated block is present, the encoding apparatus and the decodingapparatus may use the motion information as a merge candidate for thecurrent block.

The encoding apparatus may select a merge candidate which providesoptimal coding efficiency among merge candidates included in the mergecandidate list as motion information for the current block. In thiscase, a merge index indicating the selected merge candidate may beincluded in a bitstream to be transmitted to the decoding apparatus. Thedecoding apparatus may select one of the merge candidates included inthe merge candidate list using the transmitted merge index and determinethe selected merge candidate as the motion information for the currentblock. Thus, when the merge mode is employed, the motion information onthe reconstructed neighboring block and/or on the collocated block maybe used as the motion information for the current block as it is. Thedecoding apparatus may reconstruct the current block by adding theprediction block to the residual transmitted from the encodingapparatus.

In the aforementioned AMVP and merge modes, the motion information onthe reconstructed neighboring block and/or motion information oncollocated block may be used in order to derive the motion informationon the current block.

In a skip mode as another mode used for inter prediction, information ona neighboring block may be used for the current block as it is.Accordingly, in the skip mode, the encoding apparatus does not transmitsyntax information, such as residual, to the decoding apparatus, exceptfor information indicating which block motion information to be used isabout as the motion information on the current block.

The encoding apparatus and the decoding apparatus may perform motioncompensation on the current block based on the derived motioninformation, thereby generating the prediction block of the currentblock. Here, the prediction block may refer to a motion-compensatedblock generated by performing motion compensation on the current block.Further, a plurality of motion-compensated blocks may form onemotion-compensated picture.

The decoding apparatus may verify a skip flag, a merge flag, or the likereceived from the encoding apparatus and derive motion informationneeded for inter prediction, for example, information on a motion vectorand a reference picture index, accordingly.

A processing unit for performing prediction may be different from aprocessing unit for determining a prediction method and details on theprediction method. For example, a prediction mode may be determined byeach PB while prediction may be performed by each TB. Aldo, a predictionmode may be determined by each PB while intra prediction may beperformed by each TB.

Pictures encoded/decoded prior to the current picture may be stored in amemory, for example, a decoded picture buffer (DPB), and be used forprediction of the current block or current picture. Pictures availablefor inter prediction of the current block may be maintained in thereference picture list.

A P slice is a slice decoded by intra prediction, or by inter predictionusing at most one motion vector and one reference picture. A B slice isa slice decoded by intra prediction, or by inter prediction using atmost two motion vectors and two reference pictures. Here, the referencepictures may include short-term reference pictures (STRPs) and long-termreference pictures (LTRPs). Pictures may be specified by Picture OrderCount (POC) which represents display order, in which STRPs may bepictures having an insignificant difference in POC from the currentpicture and LTRPs may be pictures having a significant difference in POCfrom the current picture.

Reference picture list 0 (“L0”) is a reference picture list used forinter prediction of a P slice or B slice. Reference picture list 1(“L1”) is used for inter prediction of a B slice. Thus, L0 is used forinter prediction of a block of a P slice involved in unidirectionalprediction, while L0 and L1 are used for inter prediction of a block ofa B slice involved in bidirectional prediction.

The decoding apparatus may construct a reference picture list whendecoding a P slice and a B slice through inter prediction. Here, areference picture used for inter prediction is specified in thereference picture list. A reference picture index refers to an indexindicating a reference picture in the reference picture list.

The reference picture list may be constructed based on a referencepicture set transmitted from the encoding apparatus. The referencepicture set may include a POC of a picture used as a reference pictureand a flag (used_by_curr_pic_s0_flag) indicating whether the picture isdirectly used as a reference. Reference pictures forming the referencepicture list may be stored in the memory, for example, the DPB. Thepictures stored in the memory, that is, the pictures encoded/decodedprior to the current picture, may be managed by the encoding apparatusand the decoding apparatus.

The reference picture set may include an STRP set including STRPs and anLTRP set including LTRPs, and an initial reference picture list may beconstructed based on the STRP set and the LTRP set.

FIG. 4 illustrates an example of constructing a reference picture list.

Reference pictures may be classified based on a current picture into afirst STRP set RefPicSetStCurr0 which includes reference pictures Ref 1and Ref 2 having a smaller POC than that of the current picture Curr, asecond STRP set RefPicSetStCurr1 which includes reference pictures Ref 3and Ref 4 having a larger POC than that of the current picture, and anLTRP set RefPicSetLtCurr which includes LTRPs Ref LT1 and Ref LT2.

Here, the first STRP set RefPicSetStCurr0 includes pictures having aused_by_curr_pic_s0_flag value of 1 (delta_poc_s0 withused_by_curr_pic_s0_flag=1), and the second STRP set RefPicSetStCurr1includes pictures having a used_by_curr_pic_s1_flag value of 1(delta_poc_s1 with used_by_curr_pic_s1_flag=1).

A default reference picture list may be formed using a group of suchreference picture sets having different properties.

Referring to FIG. 4, reference picture list 0, L0, sequentially includesthe first STRP set RefPicSetStCurr0, the second STRP set 2RefPicSetStCurr1 and the LTRP set RefPicSetLtCurr.

Reference picture list 1, L1, sequentially includes the second STRP setRefPicSetStCurr1, the first STRP set RefPicSetStCurr0 and the LTRP setRefPicSetLtCurr.

A number of reference pictures to be included in a reference picturelist may be determined based on information transmitted from theencoding apparatus. For example, the encoding apparatus may construct areference picture list, determine a number of reference pictures to useand transmit information on the number of reference pictures to use, forexample, num_ref_idx_lX_default_active_minus1 where X=0 or 1, to thedecoding apparatus as a syntax element of a sequence parameter set(SPS). The decoding apparatus may use the number of reference pictures,specified by a value of num_ref_idx_lX_default_active_minus1 plus 1, asa default in a current sequence.

Further, to specify a number of reference pictures by picture or slice,the encoding apparatus may transmit extra information indicating anumber of reference pictures, for example, num_ref_idx_l1_active_minus1where X=0 or 1, through a picture parameter set (PPS) or slice header.The decoding apparatus may apply a specified value ofnum_ref_idx_l1_active_minus1 plus 1 as the number of reference picturesfor a current picture or current slice.

In inter prediction, motion compensation may be carried out using aspecified reference picture in the reference picture list constructed asabove.

In a multi-layer structure providing spatial scalability or multi-viewscalability, reference pictures in an upper layer may include referencepictures in the same layer and an inter-layer reference picture.

In this case, the inter-layer reference picture may be signaled throughinformation for identifying a layer and information for identifying areference picture. For example, if a picture in a j-th layer is presentin the same access unit as a current picture in an i-th layer and anidentifier of the picture, nuh_layer_id, transmitted in a networkabstraction layer (NAL) unit header has the same value as aRefPiclayerId value for the current picture, i being greater than j, thepicture may be determined to be used as a reference picture for thecurrent picture. The inter-layer reference picture may represent anLTRP.

RefPicLayerId, which is a value signaled through a syntax elementinter_layer_pred_layer_idc included in a slice header, means a layerthat a current layer refers to for inter-layer prediction.

Meanwhile, when a picture of a lower layer is added to a referencepicture list for an upper layer, the picture of the lower layer may besubjected to a single fixed filter and then added to the referencepicture list for the upper layer. In this case, however, codingefficiency may decrease.

When two filters are applied to a picture of the lower layer, one ormore pictures may be added to the reference picture list, and thusinformation for constructing or signaling the reference picture list mayincrease.

Accordingly, in applying a filter to a lower-layer picture, the presentinvention adaptively selects/employs one or more filters for apredetermined unit and adds one lower-layer picture to an upper-layerreference picture list, thereby enhancing coding efficiency andpreventing an increase in complexity of constructing the referencepicture list.

FIG. 5 is a flowchart illustrating a video processing method accordingto the present invention. In detail, FIG. 5 illustrates a method ofencoding and decoding a multi-layer video which uses a first-layerpicture when encoding and decoding a second-layer picture. The method ofFIG. 5 may be applied to both a video decoding method and a videoencoding method.

First, for filtering a first-layer picture, the encoding apparatus andthe decoding apparatus determine a type of a filter for the first-layerpicture (S510). The first-layer picture is decoded before the type ofthe filter is determined, in which the first-layer picture and thesecond-layer picture may be decoded by different components or modules.The encoding apparatus and the decoding apparatus may respectivelyinclude decoding modules having the configurations illustrated in FIGS.1 and 2 for decoding the first-layer picture and decoding modules havingthe configurations illustrated in FIGS. 1 and 2 for decoding thesecond-layer picture.

FIG. 6 illustrates pictures in a plurality of layers according to thepresent invention. As shown in FIG. 6, a second-layer picture may be atarget picture to be encoded and decoded, and a first-layer picture is acorresponding picture to the target picture, which may be a referencepicture of the second-layer picture.

The target picture and the corresponding picture have the same POC, forexample, a POC of 4. For prediction of the target picture, thecorresponding picture having a POC of 4 and a second-layer picturehaving a POC of 0 or 8 may be used.

The second-layer target picture to be encoded/decoded may have a sizethe same as or different from that of the first-layer correspondingpicture. The second-layer target picture and the first-layercorresponding picture may have the same size but have differentcharacteristics of signals. Thus, a first-layer reconstructed picturemay be subjected to up-sampling or re-sampling so that the pictures ofthe two layers have the same size or the signal characteristics arechanged, thereby improving prediction efficiency.

A filter may be applied for sampling, in which different filters may beused for a luma component or a chroma component.

The encoding apparatus and the decoding apparatus may determine a singlefixed filter as the filter for the first-layer picture or adaptivelyselect one or more filters for the first-layer picture.

When a fixed filter is employed for the first-layer picture, theencoding apparatus and the decoding apparatus may apply a defaultinterpolation filter to the first-layer corresponding picture.

The default filter may have a filter set for luma/chroma signalsillustrated below in Tables 1 and 2, and a phase and filter coefficientmay vary depending on a ratio in size between pictures in layers.

TABLE 1 interpolation filter coefficients phase p f_(L)[p, 0] f_(L)[p,1] f_(L)[p, 2] f_(L)[p, 3] f_(L)[p, 4] f_(L)[p, 5] f_(L)[p, 6] f_(L)[p,7] 0 0 0 0 64 0 0 0 0 1 n/a n/a n/a n/a n/a n/a n/a n/a 2 n/a n/a n/an/a n/a n/a n/a n/a 3 n/a n/a n/a n/a n/a n/a n/a n/a 4 n/a n/a n/a n/an/a n/a n/a n/a 5 −1 4 −11 52 26 −8 3 −1 6 n/a n/a n/a n/a n/a n/a n/an/a 7 n/a n/a n/a n/a n/a n/a n/a n/a 8 −1 4 −11 40 40 −11 4 −1 9 n/an/a n/a n/a n/a n/a n/a n/a 10 n/a n/a n/a n/a n/a n/a n/a n/a 11 −1 3−8 26 52 −11 4 −1 12 n/a n/a n/a n/a n/a n/a n/a n/a 13 n/a n/a n/a n/an/a n/a n/a n/a 14 n/a n/a n/a n/a n/a n/a n/a n/a 15 n/a n/a n/a n/an/a n/a n/a n/a

TABLE 2 interpolation filter coefficients phase p fC[p, 0] fC[p, 1]fC[p, 2] fC[p, 3] 0  0 64  0  0 1 n/a n/a n/a n/a 2 n/a n/a n/a n/a 3n/a n/a n/a n/a 4 −4 54 16 −2 5 −6 52 20 −2 6 −6 46 28 −4 7 n/a n/a n/an/a 8 −4 36 36 −4 9 −4 30 42 −4 10 n/a n/a n/a n/a 11 −2 20 25 −6 12 n/an/a n/a n/a 13 n/a n/a n/a n/a 14 −2 10 58 −2 15  0  4 62 −2

Table 1 illustrates 16-phase re-sampling filter coefficients for a lumasignal, and Table 2 illustrates 16-phase re-sampling filter coefficientsfor a chroma signal.

Here, phase 0 may mean a filter coefficient for a sample at an integerposition, providing an unfiltered result.

Alternatively, the encoding apparatus and the decoding apparatus mayapply an alternative filter as a fixed filter to the first-layercorresponding picture or a picture having been subjected to the defaultfilter.

The alternative filter may have a filter set including one or morefilter coefficients, similar to the aforementioned default filter. Inthis case, the filter coefficients may be determined by the encodingapparatus and signaled to the decoding apparatus.

The alternative filter may be applied to the picture having beensubjected to the default filter and refer to a filter for the sample atthe integer position corresponding to phase 0 of the default filter.Here, the alternative filter may have fixed filter coefficients, forexample, [−1, 3, 12, 3, −1]/16.

In a video supporting quality scalability (SNR scalability), thefirst-layer picture has the same size as the second-layer picture andthus may not be subjected to filtering. However, in the presentinvention, the first-layer corresponding picture may be subjected to thealternative filter so as to change signal characteristics of the firstlayer.

Here, a flag indicating whether the alternative filter is used may besignaled through at least one of a video parameter set, a sequenceparameter set, a picture parameter set and a slice header.

In another embodiment, the encoding apparatus and the decoding apparatusmay adaptively apply one or more filters to the first-layercorresponding picture, instead of the fixed filter.

For instance, in a video supporting spatial scalability, the encodingapparatus and the decoding apparatus may apply the default filter to afirst-layer picture and then always apply the alternative filter to asample at an integer position. Here, the decoding apparatus may filterthe picture by adaptively applying the two filters, the default filterand the alternative filter, without additional signaling from theencoding apparatus.

According to still another embodiment, in a video supporting spatialscalability, the encoding apparatus and the decoding apparatus apply thedefault filter to a first-layer picture. Then, the decoding apparatusmay determine based on a flag signaled from the encoding apparatuswhether to apply the alternative filter.

The encoding apparatus may apply the default filter to the first-layerpicture and then apply the alternative filter to a sample at an integerposition. After the alternative filter is applied to the sample at theinteger position, the encoding apparatus may calculate at least one ofrate-distortion optimization (RDO), a sum of absolute differences (SAD)and a sum of absolute transformed differences (SATD) to determinewhether to apply the alternative filter. If a calculation result showsthat applying the alternative filter produces a better effect thanapplying no alternative filter, the encoding apparatus may determine toapply the alternative filter to the sample at the integer position. Asignal regarding whether to apply the alternative filter, that is, aflag indicating whether to apply the alternative filter, may be signaledthrough at least one of a video parameter set, a sequence parameter set,a picture parameter set and a slice header.

In yet another embodiment, the encoding apparatus and the decodingapparatus may apply the default filter to a first-layer picture in avideo supporting spatial scalability and apply the alternative filter toa first-layer picture in a video supporting quality scalability.

In still another embodiment, the encoding apparatus and the decodingapparatus may adaptively select and apply a filter based on acorrelation between samples. That is, a correlation between samples iscalculated by specific block unit of 4×4, 8×8, 16×16, 32×32 or 64×64,and compared with a specific threshold, thereby adaptively applying afilter. Here, the correlation between the samples may be calculatedbased on the first-layer corresponding picture (reconstructed picture)or the picture having been subjected to the default filter, and thethreshold may vary depending on spatial scalability or qualityscalability.

For instance, in a 4×4 block shown in FIG. 7, the encoding apparatus andthe decoding apparatus may calculate vertical activity (VA) andhorizontal activity (HA) using Equation 1.

VA=|2R _((i,j)) −R _((i−1,j)) −R _((i+1,j))|

HA=|2R _((i,j)) −R _((i−1,j)) −R _((i+1,j))|  [Equation 1]

Here, i and j are 0 and 2.

When VA and HA are calculated by Equation 1, block activity may becalculated by Equation 2.

Block activity=(VA+HA)>>2  [Equation 2]

If the block activity is smaller than the threshold, the encodingapparatus and the decoding apparatus may apply the alternative filter tothe target block. Otherwise, that is, if the block activity is thethreshold or greater, the encoding apparatus and the decoding apparatusmay not apply the alternative filter.

The same process of calculating VA, HA and block activity and applyingthe alternative filter may be employed for an 8×8 block or largerblocks.

Alternatively, in an N×N block, the encoding apparatus and the decodingapparatus may apply the alternative filter to the target block if avertical sample and a horizontal sample satisfy Equation 3; and may notapply the alternative filter otherwise.

Abs(p[−1][N>>1−1]+p[N−1][N>>1−1]−2*p[N>>1−1][N>>1−1])<(1<<(BitDepthY−5))

Abs(p[N>>1−1][−1]+p[N>>1−1][N−1]−2*p[N>>1−1][N>>1−1])<(1<<(BitDepthY−5))  [Equation3]

Here, p[x][y] may be a sample value at a (x, y) position, Abs may be anabsolute value, and BitDepthY may be a bit depth of a luma signal.

As such, according to the present invention, either of the fixed filtersof the default filter and the alternative filter may be applied, oreither or both of the default filter and the alternative filter may beselectively applied when the first-layer picture is filtered.

When the type of the filter for the first-layer picture is determined,the encoding apparatus and the decoding apparatus may determine a targetto which the filter is applied (S520).

The encoding apparatus and the decoding apparatus may apply the fixedfilter or adaptive filter by first-layer corresponding picture, slice,coding unit (CU), prediction unit (PU), transform unit (TU) and N×Nunit.

For example, the encoding apparatus may determine whether to use thealternative filter for each picture and signal a flag indicating whetherto use the filter through a PPS. The decoding apparatus may determinebased on the signaled flag whether to use the alternative filter andperform filtering by picture.

In another embodiment, the encoding apparatus may determine whether touse the alternative filter for each slice and signal a flag indicatingwhether to use the filter through a slice header. The decoding apparatusmay determine based on the signaled flag whether to use the alternativefilter and perform filtering by slice.

Alternatively, the encoding apparatus and the decoding apparatus mayanalyze a correlation between samples in each PU and adaptively applythe default filter or alternative filter. In this case, as the encodingapparatus and the decoding apparatus may separately analyze thecorrelation between the samples and adaptively perform filtering,signaling whether to perform filtering may not be additionally needed.

For instance, a predetermined unit for adaptive filtering may bedetermined, and the default filter or alternative filter may be appliedto each predetermined unit. Here, the alternative filter may be a filterfor filtering a sample at an integer position.

When the predetermined unit is 8×8, a correlation between samples isderived by 8×8 unit for the first-layer picture, followed by adaptivefiltering.

When the type of the filter and the target of filtering are determinedin S510 and S520, the encoding apparatus and the decoding apparatusfilter the first-layer picture using a single filter or one or morefilters in combination (S530). That is, the encoding apparatus and thedecoding apparatus may filter the target of filtering using thedetermined filter.

Subsequently, the encoding apparatus and the decoding apparatus may addthe filtered first-layer picture to a second-layer reference picturelist (S540). The first-layer picture may be added to the second-layerreference picture list by a prediction module for decoding thesecond-layer picture.

When encoding and decoding the second-layer target picture, one or morefirst-layer pictures obtained via filtering may be added for use to arandom position of reference picture list 0 or reference picture list 1for the second layer.

In this case, the encoding apparatus and the decoding apparatus mayallocate a first-layer picture to a random index in the referencepicture list.

FIG. 8 illustrates POCs of first-layer and second-layer picturesaccording to the present invention. Referring to FIG. 8, a targetpicture of encoding and decoding may be a second-layer picture having aPOC of 2 and refer to a first-layer corresponding picture, a picturehaving a smaller POC than the second-layer picture, for example,pictures with a POC of 0 or 1, and a picture having a larger POC thanthe second-layer picture, for example, pictures with a POC of 3 or 4, asSTRPs. Further, the target picture may also refer to an LTRP (notshown).

FIG. 9(A) and FIG. 9(B) illustrate a reference picture list according tothe present invention. As shown in FIG. 9(A) and FIG. 9(B), when areference picture is added to a reference picture list, the encodingapparatus and the decoding apparatus may add a first-layer picture F0 toa random position.

For example, referring to FIG. 9(A), the encoding apparatus and thedecoding apparatus may construct reference picture list 0, List 0, bysequentially adding STRPs having a smaller POC (pictures with POCs of 0and 1) than that of a current target picture, STRPs having a greaterPOC, pictures with POCs of 3 and 4, than that of the current targetpicture, the first-layer picture F0, and finally an LTRP lt to List 0.

Similarly, reference pictures list 1, List 1, may be constructed bysequentially including the STRPs having the greater POC (pictures withPOCs of 3 and 4) than that of the current target picture, the STRPshaving the smaller POC (pictures with POCs of 0 and 1) than that of thecurrent target picture, the first-layer picture F0, and the LTRP lt.

In another embodiment, the encoding apparatus and the decoding apparatusmay construct a reference picture list as in FIG. 9(B). Referring toFIG. 9(B), reference picture list 0, List 0, may be constructed bysequentially including the STRPs having the smaller POC (pictures withPOCs of 0 and 1) than that of the current target picture, thefirst-layer picture F0, the STRPs having the greater POC (pictures withPOCs of 3 and 4) than that of the current target picture, and the LTRPlt.

Similarly, reference picture list 1, List 1, may be constructed bysequentially including the STRPs having the greater POC (pictures withPOCs of 3 and 4) than that of the current target picture, thefirst-layer picture F0, the STRPs having the smaller POC (pictures withPOCs of 0 and 1) than that of the current target picture, and the LTRPlt.

In still another embodiment, the encoding apparatus and the decodingapparatus may add the first-layer picture F0 last to each list. That is,second-layer reference pictures corresponding to 1, 0, 3, 4 and lt maybe added to List 0, and then the first-layer picture F0, obtained viafiltering, may be finally added to List 0. Also, the first-layer pictureF0 may be finally added to List 1 following 3, 4, 1, 0 and lt.

Meanwhile, in one embodiment, a different number of first-layer picturesmay be added to each list, and a filter applied to pictures added toeach list may be a specified fixed filter or be determined by theencoding apparatus and signaled to the decoding apparatus.

FIG. 10 illustrates a reference picture list for a P slice according toan exemplary embodiment of the present invention.

Referring to FIG. 10, a single picture F0 obtained by filtering afirst-layer picture may be added last to reference picture list 0 for aP slice. Here, pictures 1, 0, 3 and 4 may be second-layer referencepictures, and F0 may be a picture obtained by filtering the first-layercorresponding picture using one of the foregoing filtering methods.

Also, referring to FIG. 10, the encoding apparatus and the decodingapparatus may add two pictures F0 and F1, obtained by filtering afirst-layer picture, to reference picture list 1 for a P slice followingthe second-layer reference pictures. Here, F0 and F1 may be picturesobtained by filtering the first-layer corresponding picture using one ofthe foregoing filtering methods illustrated in FIGS. 5 to 7.

FIG. 11(A), FIG. 11(B) and FIG. 11(C) illustrate a reference picturelist for a B slice according to an exemplary embodiment of the presentinvention.

Referring to FIG. 11(A), the encoding apparatus and the decodingapparatus may add a single picture F0 obtained by filtering afirst-layer picture to reference picture list 0 or reference picturelist 1 for a B slice. Here, the first-layer picture F0 may be a pictureobtained by filtering the first-layer corresponding picture using one ofthe foregoing filtering methods illustrated in FIGS. 5 to 7.

Also, referring to FIG. 11(B), the encoding apparatus and the decodingapparatus may add two pictures obtained by filtering a first-layerpicture to reference picture list 0 and reference picture list 1 for a Bslice. Here, F0 and F1 may be pictures obtained by filtering thefirst-layer corresponding picture using one of the foregoing filteringmethods illustrated in FIGS. 5 to 7, which may be the same.

For example, when the encoding apparatus determines to use thealternative filter through an SATD, F0 may be a picture filtered by thedefault filter and F1 may be a picture filtered by the default filterand then by the alternative filter.

Here, regarding quality scalability, F0 may be the corresponding picturewhich is unfiltered and F1 may be a picture filtered by the alternativefilter.

Alternatively, referring to FIG. 11(C), the encoding apparatus and thedecoding apparatus may add four pictures F0, F1, F2 and F3, obtained byfiltering a first-layer picture, to reference picture list 0 andreference picture list 1 for a B slice. Here, F0, F1, F2 and F3 may bepictures obtained by filtering the first-layer corresponding pictureusing one of the foregoing filtering methods, wherein at least two ormore may be the same.

As described above, the present invention provides a video decodingmethod of adaptively applying one or more filters to a lower-layerpicture and adding the picture to a reference picture list for an upperlayer to reduce a prediction error in the upper layer and to improveencoding efficiency, and an apparatus using the same.

Accordingly, the video decoding method and the apparatus using the samemay enhance encoding efficiency without increasing the reference picturelist.

In the aforementioned embodiments, methods have been described based onflowcharts as a series of steps or blocks, but the methods are notlimited to the order of the steps of the present invention and any stepmay occur in a step or an order different from or simultaneously as theaforementioned step or order. Further, it can be appreciated by thoseskilled in the art that steps shown in the flowcharts are not exclusiveand other steps may be included or one or more steps do not influencethe scope of the present invention and may be deleted.

The foregoing embodiments include various aspects of examples. Althoughall possible combinations to illustrate various aspects may notdescribed herein, it will be understood by those skilled in the art thatvarious combinations may be made therein without departing from thespirit and scope of the invention as defined by the appended claims.Therefore, all differences, changes and modifications within the scopewill be construed as being included in the present invention.

1. A video decoding method supporting a plurality of layers, the methodperformed by a video decoding apparatus and comprising: generating areference picture list of a current block including an inter-layerreference picture; generating a prediction block of a current block inthe current picture by referencing at least one among a plurality ofreference pictures included in the reference picture list; andgenerating a reconstructed block of the current block in the currentpicture using the prediction block; wherein the inter-layer referencepicture is added to the reference picture list as being marked as along-term reference picture, wherein motion information used for theinter prediction is derived by using motion information of at least oneof temporal and spatial neighboring blocks of the current block, andwherein the temporal neighboring block is an external block locatedoutside of a block present at a position spatially corresponding to thecurrent block, within a reference picture included in the referencepicture list.
 2. A video encoding method supporting a plurality oflayers, the method performed by a video encoding apparatus andcomprising: generating a reference picture list of a current blockincluding an inter-layer reference picture; and encoding information ofthe reference picture list; wherein the inter-layer reference picture isadded to the reference picture list as being marked as a long-termreference picture, wherein motion information used for the interprediction is encoded by using motion information of at least one oftemporal and spatial neighboring blocks of the current block, andwherein the temporal neighboring block is an external block locatedoutside of a block present at a position spatially corresponding to thecurrent block, within a reference picture included in the referencepicture list.
 3. A non-transitory computer-readable medium storing abitstream generated by a video encoding method, the video encodingmethod comprising: generating a reference picture list of a currentblock including an inter-layer reference picture; and encodinginformation of the reference picture list; wherein the inter-layerreference picture is added to the reference picture list as being markedas a long-term reference picture, wherein motion information used forthe inter prediction is encoded by using motion information of at leastone of temporal and spatial neighboring blocks of the current block, andwherein the temporal neighboring block is an external block locatedoutside of a block present at a position spatially corresponding to thecurrent block, within a reference picture included in the referencepicture list.